Remove VerboseModule so zarr.storage can be serialized

changes/4029.bugfix.md

@@ -0,0 +1,8 @@
+`zarr.storage` is no longer replaced by a `ModuleType` subclass at import time.
+The subclass only existed to warn about the long-deprecated
+`zarr.storage.default_compressor` attribute, but serializers that dispatch on
+the exact type of an object (such as `cloudpickle`, which Dask uses by default)
+could not serialize the module and raised
+`TypeError: cannot pickle 'VerboseModule' object`. Setting
+`zarr.storage.default_compressor` no longer emits a deprecation warning; use
+`zarr.config` to configure `array.v2_default_compressor` instead.

pyproject.toml

@@ -109,6 +109,7 @@ homepage = "https://github.com/zarr-developers/zarr-python"
 # pins deliberately, e.g. via dependabot or `uv lock --upgrade`.
 [dependency-groups]
 test = [
+    "cloudpickle==3.1.2",
     "coverage==7.16.2",
     "pytest==9.1.1",
     "pytest-asyncio==1.4.0",

src/zarr/storage/__init__.py

@@ -1,9 +1,3 @@
-import sys
-import warnings
-from types import ModuleType
-from typing import Any
-
-from zarr.errors import ZarrDeprecationWarning
 from zarr.storage._common import StoreLike, StorePath
 from zarr.storage._fsspec import FsspecStore
 from zarr.storage._local import LocalStore
@@ -26,20 +20,3 @@
     "WrapperStore",
     "ZipStore",
 ]
-
-
-class VerboseModule(ModuleType):
-    def __setattr__(self, attr: str, value: Any) -> None:
-        if attr == "default_compressor":
-            warnings.warn(
-                "setting zarr.storage.default_compressor is deprecated, use "
-                "zarr.config to configure array.v2_default_compressor "
-                "e.g. config.set({'codecs.zstd':'numcodecs.Zstd', 'array.v2_default_compressor.numeric': 'zstd'})",
-                ZarrDeprecationWarning,
-                stacklevel=1,
-            )
-        else:
-            super().__setattr__(attr, value)
-
-
-sys.modules[__name__].__class__ = VerboseModule

tests/test_store/test_core.py

@@ -1,8 +1,10 @@
 import tempfile
 from collections.abc import Awaitable, Callable, Generator
 from pathlib import Path
+from types import ModuleType
 from typing import Any, Literal
 
+import cloudpickle
 import pytest
 from packaging.version import parse as parse_version
 
@@ -434,3 +436,15 @@ def test_different_open_mode(tmp_path: Path) -> None:
         match="Store is not read-only but mode is 'r'. Unable to create a read-only copy of the store. Please use a read-only store or a storage class that implements .with_read_only().",
     ):
         zarr.open_array(store=zip_store, path="a", zarr_format=2, mode="r")
+
+
+def test_storage_module_is_picklable() -> None:
+    # `zarr.storage` used to be swapped for a `VerboseModule` subclass of
+    # `types.ModuleType` in order to warn about the long-deprecated
+    # `zarr.storage.default_compressor` attribute. cloudpickle dispatches on the
+    # exact type of an object, so the subclass was not registered and
+    # serializing the module raised `TypeError: cannot pickle 'VerboseModule'
+    # object`, which broke passing `zarr.storage.ObjectStore` around with Dask.
+    # See issue #4029.
+    assert type(zarr.storage) is ModuleType
+    assert cloudpickle.loads(cloudpickle.dumps(zarr.storage)) is zarr.storage

tests/test_v2.py

@@ -20,7 +20,6 @@
 from zarr.core.dtype.wrapper import ZDType
 from zarr.core.group import Group
 from zarr.core.sync import sync
-from zarr.errors import ZarrDeprecationWarning
 from zarr.storage import MemoryStore, StorePath
 
 
@@ -225,11 +224,6 @@ def test_v2_non_contiguous(numpy_order: Literal["C", "F"], zarr_order: Literal["
         assert (sub_arr).flags.c_contiguous
 
 
-def test_default_compressor_deprecation_warning() -> None:
-    with pytest.warns(ZarrDeprecationWarning, match="default_compressor is deprecated"):
-        zarr.storage.default_compressor = "zarr.codecs.zstd.ZstdCodec()"  # type: ignore[attr-defined]
-
-
 @pytest.mark.parametrize("fill_value", [None, (b"", 0, 0.0)], ids=["no_fill", "fill"])
 def test_structured_dtype_roundtrip(fill_value: float | bytes, tmp_path: Path) -> None:
     a = np.array(